What says the actual doc :

$builder->add('users', 'entity', array(
'class' => 'Acme\HelloBundle\Entity\User',
));

This kind of 'class' reference products an error : "Expected argument of type "object", "string" given"

The working way i found to refer to the class :

$builder->add('users', 'entity', array(
'class' => 'AcmeHelloBundle:User',
));